Spark plug insulators containing stannic oxide



Unite P tented SPARK PLUG INSULATORS CUNTAINING STANNTC OXIDE Harry G. Schnrecht, Detroit, Mich assignor to Champion Spark Plug Company, Toledo, Ohio, a corporation of Delaware No Drawing. Application .l'uly 23, 1958 Serial N0. 751,175

6 Claims. (Cl. 106-46) This invention relates to spark plug insulators containing stannic oxide, and, more particularly, to such insulators produced by firing particular compositions of oxide or oxide-forming materials including stannic oxide.

The use of tin oxide in making different refractory bodies has previously been suggested, for example, bodies suitable for use in Contact with molten glass, and bodies having a high dielectric constant (in excess of one thousand in all instances) suitable for use as electro-mechanical devices such as piezo electric crystals.

Ceramic insulators having a dielectric constant greater than about 10 are completely unsuited for spark plug uses. Relatively high voltages are conducted interior- 1y through the electrode system of spark plug insulators. The air surrounding such insulators is ionized to a substantial extent by this electric current. The ionization of the air surrounding the insulator provides a potential path for spark discharge, for example from the metal connection at the top of an ordinary automotive spark plug to the grounded portion of the spark plug, or to a metallic component of an associated automobile engine. When the dielectric constant of the insulator is as high as about 10 such ionization occurs to an extent sufficient that discharge in this manner occurs to the exclusion of the desired discharge across the spark gap of the plug. Ceramic insulators can be produced from pure stannic oxide and also from mixtures of stannic oxide with minor amounts of fluxing ingredients. Such insulators, however, have properties which make them completely unsuited fOijuse' as spark plug ceramics. For example, a'j spark plug insulator is subjected to high temperatures inf service, and must be effective at such high temperatitres, as we'll as at low temperatures which are encountered in a cold engine and all intermediate temperatures. To be suited for spark plug use, a ceramic,

the form of a cylinder about /2" in diameter and approximately /2" in length must have an electrical resistance, at 1350 F. and at lower temperatures, of at least 1 megohn'r. Since it is a general characteristic of ceramic materials that their resistance decreases with increases in temperature, a particular ceramic can be shown to be suitable for spark plug use by determining that a cylinder thereofof the indicated size has an electrical resistance of at least 1 megohm at a temperature of 1350" F. or higher, or a given ceramic can be shown to be unsuited for spark plug use by a determination that such a cylinder thereof has a resistance of less than 1 megohm at a temperature of 1350 F. or lower. lt -has been determined that such a cylinder produced from pure stannic oxide or from a mixture of 96.5 parts of s'tanuic oxide with 1.5 parts of whiting, 2.0 parts of talc and 1.5 parts of bentonite has a resistance of less than 0.1 megohm at a temperature of 1000 F. Neither of such insulators, therefore, is suited for spark plug use. This suggests that stannic oxide would be an undesirable constituent in a ceramic batch for use in the preparation of a spark plug insulator becausestannic oxide in an appreciable amount would be expected to have an undesirable effect upon electrical resistance, at least at elevated temperatures.

The terms percent and parts are used herein and in the appended claims to refer to percent and parts by weight, unless otherwise indicated.

The present invention is based upon the discovery that it is possible to produce sintered ceramic spark plug insulators by firing mixtures consisting essentially of alumina and stannic oxide, in certain proportions, and that the resulting insulators not only have the dielectric properties requisite for spark plug use, but also have thermal conductivity characteristics which are at least equivalent to presently used spark plug ceramics,- and are resistant to corrosion by compounds of lead and other chemicals which are present in the firing chambers of present-day internal combustion engines. 7

According to the invention, a sintered ceramic spark plug insulator is provided. Such an insulator is pro duced by firing a blend of refractory materials consisting essentially of from about 19 parts to about 81 parts of 8:10 and from about 81 parts to about 19 parts of A1 0 Insulators produced from such blends have the previously indicated properties, and limited amounts of other materials can be added to the blends, as will subsequently be discussed in more detail, without appreciably affecting such properties of insulators which are produced therefrom. The suitability of various compositions consisting of stannic oxide and alumina, in proportions as set forth above, for use in the manufacture of electrical insulators for spark plug purposes was demonstrated by preparing several insulators, and testing the properties of each. Representative examples of such compositions, and the properties of insulators produced therefrom, are given in the following example, which, like the subsequent example, is to be construed as illustrative only, and not as limiting the invention.

EXAMPLE 1 Several blends consisting of stannic oxide and alumina were we mmed to substantial uniformity (usually for 18 hours), and were then dried and mixed with 4 percent of parafiin wax, based upon the weight of dry in gredients, dissolved in carbon tetrachloride. The wait was thoroughly mixed with the dry ingredients, and the carbon tetrachloride volatilized, leaving the wax uniformly dispersed throughout the composition. Test cylinders about /2 in diameter and approximately /2' in length were then pressed from each mixed composition, using a total pressure of 10,000 pounds. The cylinders were then fired to Cone 31. V I The bodies so prepared were tested to determine their suitability for use as spark plug insulators. The electrical resistance of the /2" cylinders was determined at l200 F. or 1460 F., and, in some instances, at 1500 As has been indicated above, a ceramic body having a resistance (when in the form of such a cylinder) of at least 1 megohm at 1350" F. or higher is considered satisfactory for spark plug purposes. In addition, all insulator having a resistance of at least about 10 megohms at 1200 F. is considered satisfactory for spark plug purposes. 7 p Another standard test was conducted to compare the thermal diffusivities of insulators produced from the different compositions: The test involved placing a crystal of citric acid on one end of the sintered /2" cylinder, and immersing the other end of the cylinder in a 600 F. metal bath to a depth of about /s"-. The numberof seconds required for heat conducted through theinsuldto'r to melt the citric acid crystal, which melts at 307. 4 F., is reported in the tables herein as thermal diffusivity, and is an inverse function of thermal conductivity. T'O

, 3 r be at least comparable with presently used spark plug insulators in this respect, a ceramic insulator should have a thermal difiusivity, so measured, not greater than 4 herein, entries under the heading Dye Test are Mat, which means matured; U.F., which means underfired, and SUF, which means slightly underfired.

about 30 seconds. Ideally, a ceramic body should have a thermal diffusivity not greater than about seconds.

A low thermal diffusivity time, or a high thermal conductivity, is an important characteristic of a spark plug insulator. high temperatures in the firing chamber of an internal combustion engine. The insulator is heated to some temperature which depends, in the main, upon the mean efiective firing chamber temperature, the thermal conductivity of the ceramic insulator and the design of the ceramic insulator and of the spark plug of which it constitutes a part. In general, high output present-day engines, when operated at or near full load, necessitate the use of spark plugs having thermal difiusivity times of seconds or less in order to avoid heating of the 30 ceramic insulator, in service, to a temperature sufficiently In service, such an insulator is subjected to 20 Table II, below, presents typical test results obtained with bodies produced from blends consisting essentially of A1 0 and SnO in the previously indicated proportions. It will be observed from the results presented in Table II that substantial amounts of ZrO can be employed in a blend used to produce an insulator according to the invention without adversely affecting the desirable characteristics of electrical resistance at elevated temperature and thermal diffusivity, and that limited amounts of whiting, talc and bentonite can be similarly added without adverse effect. All of the bodies reported in Table I, above, and in Table II, below, have been tested and found to be unexpectedly resistant to corrosion by lead and other compounds which are usually present in automotive and aviation fuels.

Table II Composition in Parts Thermal ElcctricResistancc Difiusivity (Megohms) Dye Body No. Time in Test Seconds A1 0; 8110; Zl'Oz Whiting Talc Bentonite 1,200 F. 1,400 F. 1,500 F.

57. 9 19. 3 19. 3 23. 8 48 36 15 SUF 57. 9 19. 3 19. 3 1. 5 2. 0 1 5 22. l) 100 23 Mat. 38. 6 38. 6 19. 3 t 29. 4 U.F. 19. 3 57. 9 19. 3 30. 6 3O U.F, 19. 3 38. 6 38. 6 1. 5 2- 0 1. 5 27. 8 11.5 Mat. 77. 2 19. 3 1. 5 2. 0 1. 5 18.6 100+ 100+ 100+ Mat. 57. 9 88. 6 1. 5 2.0 1. 5 18. 3 100+ 50 38 Mat. 38. 6 67. 9 1. 5 2.0 l. 5 17. 8 3. 1 SUF high that it Will cause premature ignition of a fuel-air mixture. Insulators having thermal diifusivity times of 20 seconds or less are ideal for use in such engines because more latitude is given a spark plug engineer in designing a plug which will operate satisfactorily without causing such premature ignition.

The insulators produced as described above were also subjected to tests to determine the adequacy of firing. The test involved application to each insulator of a dye solution, and visual examination of the so-treated insulator for evidence of dye absorption. Any insulator which absorbed dye was considered to be underfired, meaning that a higher firing temperature should be used, or that some fluxing material should be mixed with the composition to enable maturation under Cone 31 firing conditions. Insulators which did not absorb dye were either matured or overburned. These two states were distinguished by a specific gravity test, insulators having the highest specific gravity being properly matured and insulators which did not absorb the dye solution, but which had a lower specific gravity, being overfired.

Typical test results for bodies produced from blends consisting of alumina and stannic oxide, in the proportions indicated above, are presented in Table I, below. In Table I the composition of each blend is given, as well as the thermal difiusivity time, measured as discussed above, the electrical resistance at one or more of the indicated elevated temperatures, and, under the heading Dye Test, the result of the above-described test for adequacy of a Cone 31 firing. In the tables cept that 19.3 parts, or, in some cases, even fewer parts,

of T102, CeO BaO or SrO were substituted for the ZrO insulators which were produced by a Cone 31 firing were found to have a thermal difiusivity time ranging from 45 seconds to 107 seconds, and were, therefore,

completely unsuited for use as spark plug insulators.

When a similar substitution of SiO for ZrO was made in the blend reported in Table II for producing body No. 5, the insulator was found to have a thermal diffusivity time of 49 seconds, and, therefore, to be unsuited for use as a spark plug insulator.

Minor amounts of whiting, talc and bentonite were also used in the blends from which bodies 6, 9, 10, l1 and 12 were produced. The bentonite was used principally as a combined binder and plasticizer, and was effective principally while the blend was in an unfired condition. It functioned as a binder for the unfired bodies, and, in its capacity as a plasticizer, facilitated the handling and processing of the unfired material. Bentom'te, or other material which acts in a similar manner as a plasticizer, as a binder, or as a combined plasticizer and binder, is preferably employed in producing an insulator according to the invention, and, most desirably, to the extent of from about 1 percent to about 2 percent, based upon the total weight of the blend, or ceramic batch. Both whiting and talc, as used in the blends or batches used in preparing the bodies reported in Table II, above, act as fluxes, tending to reduce the cone number designation of the firing required to effect maturation. Such a flux, or a different flux, is preferably employed in producing an insulator according to the invention, and, most desirably, to the extent of from about 1 percent to about percent, based upon the total weight of the blend or ceramic batch. It is also preferred that the flux include an alkaline earth compound which, after firing, is in the form of the oxide, and most desired that it include, in addition to the alkaline earth compound, a magnesium silicate.

While reference has been made herein to stannic oxide, or SnO as well as to alumina, or A1 0 other tin oxides can be used in place of stannic oxide, or other tin compounds which revert to an oxide during firing, or compounds other than alumina which revert to alumina during firing can similarly be employed. Substantially the same properties result in the completed insulator. In addition, an entire insulator can be produced from a blend, as is disclosed in detail in the foregoing examples, or only a portion thereof can be produced from the blend, for example the firing tip, or the surface of the firing tip.

This is a continuation-in-part of application Serial No. 371,073, filed July 29, 1953, which was, in turn, a continuation-in-part of application Serial No. 69,409, filed January 5, 1949, both now abandoned.

It will be apparent that various changes and modifications can be made from the specific details set forth herein without departing from the spirit and scope of the attached claims.

What I claim is:

1. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 19 parts to about 81 parts of SnO and from about 81 parts to about 19 parts of A1 0 2. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 19 parts to about 81 parts of Sn0 and from about 81 parts to 6 about 19 parts of A1 0 and including, in addition, based upon the total weight of the blend, from about 1 percent to about 5 percent of a flux.

3. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 19 parts to about 81 parts of SnO and from about 81 parts to about 19 parts of Al O and including in addition, based upon the total weight of the blend, from about 1 percent to about 5 percent of a flux comprising an alkaline earth compound which, after firing, is in the oxide form.

4. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 19 parts to about 81 parts of SnO and from about 81 parts to about 19 parts of A1 0 and including, in addition, based upon the total weight of the blend, from about 1 percent to about 5 percent of a flux comprising a magnesium silicate and an alkaline earth compound which, after firing, is in the oxide form.

5. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a blend consisting essentially of from about 19 parts to about 81 parts of SnO and from about 81 parts to about 19 parts of A1 0 and including, in addition, based upon the total Weight of the blend, from about 1 percent to about 5 percent of a flux comprising a magnesium silicate and an alkaline earth compound which, after firing, is in the oxide form, and from about 1 percent to about 2 percent of a plasticizer.

6. A ceramic spark plug insulator, at least the surface of which, at the firing end, is produced by sintering a. blend consisting essentially of from about 19 parts to about 81 parts of SnO and from about 81 parts to about 19 parts of A1 0 and including, in addition, based upon the total weight of the blend, from about 1 percent to about 5 percent of a flux comprising a magnesium silicate and an alkaline earth compound which, after firing, is in the oxide form, and from 1 percent to 2 percent of bentonite.

References Cited in the file of this patent UNITED STATES PATENTS 2,255,203 Wiegand Sept. 9, 1941 2,413,441 Feichter Dec. 31, 1946 2,577,369 Schurecht Dec. 4, 1951 

1. A CERAMIC SPARK PLUG INSULATOR, AT LEAST THE SURFACE OF WHICH, AT THE FIRING END, IS PRODUCED BY SINTERING A BLEND CONSISTING ESSENTIALLY OF FROM ABOUT 19 PARTS TO ABOUT 81 PARTS OF SNO2, AND FROM ABOUT 81 PARTS TO ABOUT
 1. PARTS OF AL2O2. 